1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus and, more particularly, to the invention of positioning in a direction which perpendicularly crosses a sheet conveying direction of a sheet which is conveyed to an image forming unit.
2. Description of the Related Art
Hitherto, among image forming apparatuses such as an electrophotographic printer and the like, there is an apparatus in which a sheet in which an image has been formed on one side is reversed and conveyed again to an image forming unit, thereby forming an image onto a back side of the sheet. Such an image forming apparatus has a sheet conveying apparatus for reversing the sheet in which the image has been formed on one side and conveying the sheet again to the image forming unit (hereinafter, such an apparatus is referred to as a sheet re-conveying apparatus).
In such a conventional sheet re-conveying apparatus, when the sheet is conveyed again to the image forming unit, there is a case where the sheet is obliquely moved during the conveyance or the position is deviated, so that when the image is formed onto the back side, the image is deviated from a predetermined position on the sheet. This is because in the case of forming the image onto the second side (back side), a conveying path which is used until the image is formed onto the sheet is longer than that for the first side and the motion of the sheet is influenced by an eccentricity of a conveying roller or a difference between pressing forces in the conveying path or by a resistance of a conveying surface of a conveying guide. Consequently, the sheet is slightly obliquely moved during the conveyance or the position of the sheet in the direction (hereinbelow, referred to as a width direction) which perpendicularly crosses the sheet conveying direction is deviated from a reference position.
To prevent such an oblique motion or a positional deviation of the sheet, for a period of time until the image is formed onto the second side after the image was formed onto the first side, it is necessary to adjust the sheet position so that the position of the image coincides with that of the sheet.
As such a sheet position adjusting method, for example, there is a construction in which a reference guide is arranged in one end portion (of the re-conveying path for conveying the sheet again to the image forming unit) in the direction (hereinbelow, referred to as a width direction) which perpendicularly crosses the conveying direction of the sheet. There is a technique for performing the positioning in the sheet width direction (hereinbelow, referred to as a lateral registration correction) by conveying the sheet while pressing a side edge of the sheet to the reference guide. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. 2000-233850.
FIGS. 9A, 9B, and 9C are top views illustrating a construction of the re-conveying path of the conventional sheet conveying apparatus having a lateral registration correcting unit for making the lateral registration correction of the sheet by such a reference guide. An example of the conventional sheet conveying apparatus will be described with reference to FIGS. 9A to 9C.
A lateral registration correcting unit 23 includes: a reference guide 24 having a reference surface 24a arranged along the sheet conveying direction; an oblique feeding roller pair 11A having an oblique feeding roller 11 and an oblique feeding rolling member (not shown) which is come into pressure contact with the oblique feeding roller 11; and a lower conveying guide 27 for guiding the lower side of the sheet. A rotary axis of the oblique feeding roller is arranged in the direction which perpendicularly crosses the reference surface 24a. A rotary axis of the oblique feeding rolling member is arranged so as to have an inclination of about 5° to 15° from the direction which perpendicularly crosses the reference surface 24a. The oblique feeding rolling member has a drum shape. Owing to such a structure, the sheet is sandwiched between the oblique feeding roller 11 and the oblique feeding rolling member and conveyed toward the reference surface 24a. Since the reference surface 24a is scraped by a sheet edge portion at the time of the sheet passage, the reference surface 24a is reinforced by forming a plurality of reference pins 24h made of a metal.
Subsequently, the lateral registration correcting operation of the lateral registration correcting unit 23 having such a construction will be described.
As illustrated in FIG. 9A, a sheet S in which an image has been formed on one side is conveyed from conveying rollers 3g provided on the upstream of the lateral registration correcting unit 23 toward the lateral registration correcting unit 23. After the sheet S reached the oblique feeding roller pair 11A, the sheet S is sandwiched between the oblique feeding roller pair 11A and conveyed. The sheet is conveyed while it is drawn toward the reference surface 24a side by the oblique feeding roller pair 11A. Subsequently, when a rear edge of the sheet S escapes from the conveying rollers 3g, the sheet S receives the resistance of the lower conveying guide 27 and rotates so that the sheet rear edge approaches the side of the reference surface 24a as illustrated in FIG. 9B. By the rotation, the sheet S is come into contact with the reference pins 24h and rotates by its reaction force so that a sheet front edge approaches the side of the reference surface 24a. 
Thus, as illustrated in FIG. 9C, the sheet S is aligned to a position along the reference surface 24a, the oblique motion of the sheet is corrected, and further, a position in the width direction of the sheet is positioned to a reference position. After the lateral registration correction was made to the sheet S by the lateral registration correcting unit 23 as mentioned above, the sheet S is conveyed again to the image forming unit (not shown) through intermediate rollers 3d. 
As mentioned above, in the lateral registration correcting unit 23, the sheet S is rotated by the oblique feeding roller pair 11A and the resistance of the lower conveying guide 27, and thereafter, the sheet S is rotated along the reference surface 24a while using the reference pins 24h as pivot points.
According to such a lateral registration correcting method, a conveyance distance until the sheet is aligned along the reference surface 24a after the rear edge of the sheet escaped from the conveying rollers 3g is short and an efficiency of the positioning (the oblique motion correction) is high. Therefore, even when the sheet is conveyed again, the positional deviation of the sheet can be certainly corrected and the sheet can be conveyed.
Among the image forming apparatuses each having such a sheet re-conveying apparatus, there is an apparatus which forms images onto two types of sheets of different sizes such as sheet of a letter size and sheet of an A4 size.
In such a case, for example, if the reference surface 24a of the reference guide 24 of the re-conveying path is set to the lateral registration correcting position for the sheet of the letter size and it is intended to make the lateral registration correction to the sheet of the A4 size by the same reference surface 24a as that of the sheet of the letter size, the following problem occurs.
When the lateral registration correction of the sheet of the A4 size is made, an amount in the width direction of the sheet which is necessary for making the sheet of the A4 size come into contact with the reference surface 24a is equal to 3 mm [=(a width of sheet of the letter size)−(a width of sheet of the A4 size)]. Further, when a deviation amount of the sheet during the conveyance until the sheet reaches the re-conveying path is assumed, the apparatus has to be set so that the sheet is moved to the reference surface 24a by up to about 5 mm. To draw the sheet to the reference surface 24a by about 5 mm as mentioned above, it is necessary to increase a conveying force of the oblique feeding roller pair 11A. For this purpose, a nip pressure of the oblique feeding roller pair 11A has to be set to be high. However, if the nip pressure of the oblique feeding roller pair 11A is increased as mentioned above, in the case where the sheet is a sheet having the letter size and a small rigidity, a drawing force to the reference surface 24a which is applied by the oblique feeding roller pair 11A becomes too strong. If the drawing force to the reference surface 24a is too strong, as illustrated in FIG. 10, the sheet S is deflected. If the sheet S is sandwiched between the intermediate rollers 3d on the downstream in the deflected state, the position in the width direction of the sheet is deviated. When the drawing force is too strong, the reference surface 24a made of a resin is scratched by the sheet edge portion. If the reference surface 24a is scratched, such a scratch becomes a conveyance resistance to the sheet edge portion and a sheet jam is caused by the scratch.
To avoid such a problem, hitherto, there is such an apparatus that the switching of the position in the width direction of the reference guide 24 is performed by using a driving force which is applied by a motor or the like. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. H08-292612.
FIG. 11 is a plan view for describing a position adjusting mechanism of the reference guide 24 in such a conventional sheet re-conveying apparatus. In FIG. 11, a block driving plate 28 is fixed to the reference guide 24 and has a rack portion (not shown). A pinion gear 29 is in engagement with the rack portion (not shown) of the block driving plate 28.
A transfer gear 30 is driven by a driving motor (not shown). A rotating force of the driving motor is transferred to the transfer gear 30 and the pinion gear 29. The rack portion (not shown) is moved by the pinion gear 29, thereby moving the reference guide 24 in the width direction through the block driving plate 28.
When the lateral registration correction of the sheet is made, the position of the reference surface 24a is adjusted through the rack and pinion gear according to a rotation amount of the driving motor. By switching the position in the width direction of the reference guide 24 by using the motor as a driving source as mentioned above, the lateral registration correction of the sheets of various sizes can be made.
However, in the conventional sheet re-conveying apparatus having the construction in which the reference guide 24 is moved in the width direction by the rack and pinion gear, the reference guide 24 rattles in the width direction by a backlash of the gear, the position is not settled, and a deviation in the width direction of the reference surface 24a of an amount corresponding to the backlash occurs. There is, consequently, such a problem that print precision in the width direction of the sheet deteriorates.